482 THE POPULAR SCIENCE MONTHLY 



sea forms, habituated to lower temperatures. When the amelioration 

 of the earth's climate took place, near the beginning of the Mesozoic 

 era, they found a free field on the coasts, and at once took possession. 

 In the epoch of the Middle Triassic they had already become widely 

 distributed, but as yet had formed no known reefs. 



The distribution of the cephalopods in time shows a strong contrast 

 to that of the corals. There is an unbroken genetic series of ammonoids 

 and nautiloids from the Coal Measures, through the Permian, and ex- 

 tending into the Lower Triassic, several genera ranging through the 

 interval. This does not necessarily mean that the cephalopods were 

 hardier, for they probably were not. But they were very widely dis- 

 tributed, and must have lived on in some region, or regions, where 

 great catastrophe had little or no effect, and by their superior facility 

 in locomotion got back into the regions affected by glaciation, when the 

 temperature of the seas had risen again. 



Mesozoic Climates of the West Coast 



Since corals are wholly unknown in the Lower Triassic, and since 

 the flora of that epoch is as yet little known, it is not possible to deter- 

 mine the temperature of either the land or the water. It is, however, 

 certain that the oceanic temperature in India, in western America and 

 in northern Siberia, was the same, for there is a remarkable similarity 

 of the cephalopod faunas in all three regions. 



It is also known that in the Permian and the Lower Triassic a dry 

 climate prevailed over large areas, for products of desiccation, such as 

 gypsum and saline deposits are common in many parts of the world, 

 and even in regions that are now rainy, as in western Europe. 



In the Upper Triassic there are great limestone masses and coral 

 reefs in the Alps, the Himalayas and in California, with many species 

 common to the three regions. Certainly the epoch of the Tropites 

 ■subbullatus fauna was tropical up as far as Shasta County, California, 

 for there reefs of Astrseidse are extensive. We may even be justified in 

 assuming that the isotherm of 74° F. extended that far north. Al^o 

 in the Blue Mountains of northeastern Oregon there are coral reefs in 

 the Upper Triassic, but no Astraaidas were found in them, only extinct 

 genera. This outlying occurrence may correspond to the isotherm of 

 68° F., in which now corals may form reefs, but Astragidse can not 

 flourish. 



After the formation of the coral reefs in northern California and 

 Oregon the facies changed suddenly from limestones to clay shales, 

 and with this came an abrupt change in the marine fauna. The Indian 

 types of cephalopods disappeared entirely, and in their stead came in a 

 fauna of which the home seems to have been the boreal region. Pseudo- 

 monotis oclwtica was the commonest species in this fauna, and was 



